Yttrium as a heavy rare earth is widely used in rare earth-aluminum alloys, functional ceramics, lasers, phosphors, and other fields.
However, because yttrium has an ionic radius (0.88 Å) close to erbium (0.881 Å) of the lanthanide series, it possesses very similarly chemical properties to the lanthanides, specifically for heavier lanthanides, thus is difficult to be separated from the others.
According to existing processes, yttrium is mainly separated by an extraction and separation method using naphthenic acid (NA) as major extractant. The disadvantages of naphthenic acid method emerged for long-term industrial practices including: {circle around (1)} As a byproduct of the oil industry, naphthenic acid contains extremely unstable ingredients; {circle around (2)} naphthenic acid has a relatively high pKa value of 7.57. It is prone to react with high valence impurity metals such as iron (III), Th (IV), titanium (IV), zirconium (IV) to form solid naphthenates under high operational pH, thus leading to severe emulsification and loss of the extractants; {circle around (3)} naphthenic acid reacts slowly with iso-octanol as a phase modifier, thereby leading saponification value to continually decrease and requiring regular supplement or even replacement of fresh extractants, which can produce a large amount of waste organic phases. Additionally, like those processes using other acidic extractants, the naphthenic acid process also requires saponification with alkalis (such as sodium hydroxide, aqueous ammonia, etc.) to form organic salts prior to extraction. In back-extraction process, it further consume a large amount of hydrochloric acid [Deqian Li, Chemical Engineering Problems in Hydrometallurgical Industry of Rare Earths, Progress in Chemistry, 7(3), 209-213, (1995)]. According to statistics, 6.5 to 8 moles of hydrochloric acid and 6 moles of alkali would be consumed per mole of yttrium oxide produced. [Jianshe Han, Qingyu Yang and Xiang Ye, The capriccio of rare earth extraction and separation, National Symposium on Rare Earth Chemistry and Metallurgy, 2014]. Chinese Invention Patent No. 99118261.8 disclosed a liquid-liquid extraction method for separating high-purity yttrium which used a mixture of sec-octylphenoxy substituted acetic acid (CA12) and a monobasic phosphoric (phosphonic) acid or its monothio derivatives (such as P204, P507, Cyanex 272, Cyanex302, etc.) as extractant, which contributed to solve the emulsification problem of naphthenic acid system. However, the above-described method still has a problem of concentration decrease of organic phase. Chinese Invention Patent No. CN201210290516 disclosed a non-saponification process for extracting and separating rare earths, which used a composite solvent as a composite extractant obtained by mixing an acidic phosphonic (phosphorus) extractant such as P507 or P204 with an amine extractant such as N235. It directly extracted and separated rare earths without saponification, but still required a back-extraction with inorganic acids. In all the above-described extraction processes acids or alkalis are used, so that the extraction is carried out under high acidic (low pH) conditions, thus lowering the extraction efficiency and causing environmental contamination with acids or alkalis.